Definition
A visible warning indicator on a navigation instrument that appears when the signal being received is too weak, unreliable, or absent, telling the pilot that the displayed information should not be trusted for navigation.
Plain English
A small flag or marker that pops into view on the instrument face to warn you that what you're seeing on that instrument is not reliable right now.
Context Anchor
Seen on VOR navigation indicators and similar cockpit displays when the navigation signal or instrument indication is not dependable.
Derivation
From 'alarm,' meaning a warning of danger or trouble, and 'flag,' a small visible marker. Together: a marker that warns you something is wrong. The word fits exactly what the indicator does.
Why Pilots Care
Using navigation data while an alarm flag is displayed can produce large course errors and loss of situational awareness.
Intuition Check
An alarm flag is not usually a ringing alarm. It is a visual warning on the instrument. If the flag is showing, treat the indication as unreliable.
Example Sentence 1
As the aircraft flew further from the VOR station, the alarm flag appeared, indicating the signal was no longer strong enough for reliable navigation.
Example Sentence 2
During the ILS approach the pilot watched for an alarm flag that would indicate loss of usable signal.